Short duration neutron pulse generating system



Jan. 3,1196? 5. FUNFER ETAL 3,296,442

SHORT DURATION NEUTRON PULSE GENERATING SYSTEM Filed Oct. 17, 1962 .7nvenfors:

comes: greater as the distance increases.

United States i Patent 3,296,442 SHORT iDUlRA'IION NEUTRON PULSEGENERATING SYSTEM Ewaldil Fiinfer and Benedikt Kronast, Munich, Germany,assignprs; to Institut fur. Plasmaphysik. Gesellschaft mit beschrankterHaftung, Munich-Garching, Germany Filed Oct. 17,1962, Ser. No. 231,257Claimsxpriority, applicatizon Germany, Oct. 25, 1961.

17 Claims. c1.2s0--s4.5

s This invention relates to an arrangement for the productionof veryshort neutron pulses of high intensity and to a method of operating thisarrangement. In particular,

l j thelyinvention relates to a tube which delivers neutrons and Iiniwhich.deuterium-ionsare accelerated by means of a high voltage and arecaused to hit a deuterium and/ or i tritium ;;target, so that high speedneutrons are formed kvmis. applied. The spacing of the acceleratingelectrodes .is limited at the. lower end by the minimum distance atwhich there is a risk of field emission and at the upper end by thetendency toward fiashover as a result of colljision ionization betweenelectrons and ions, which be- The conditions in the tube correspond tothe region on the left hand side of the minimumin thePaschen curve(dependence of the ignition voltage. of a gas discharge on the productof pressure and electrode spacing).

Such arrangements are described in: Rev. Sci. Instr. 30/315/59;31/235/60; 311/241/60; Nucleonics l8/No. 12, pages 69-76 (1960).

. In;addition,xa tube of this type is known, the anode of which consistsof a hemispherical metal part which is fused} to a glass tube: At theother end of the glass tube is a target arrangement. and at a shortdistance in front of which a grid electrode is arranged. This gridelectrode is negatively. biased in relation to the target and prevents Ithe secondary electrons formed at the: target from reaching]; the.anode. Mounted directly on the anode cap is a pulse transformer with anopen magnet core which supplies the cap with a high-voltage pulse whichionizes the gasrfilling in the tube. The considerable stray magneticfields; which increase the probability of ionization in the fregion: ofthe anode, appear to be essential to the functioning of the tube.. Sucha tube is described in Rev. Sci.

. the. gas. filling and an acceleration of the ions produced occurs, yeta low-pressure high-voltage disruptive discharge .does. not. Disruptivedischarges are highly undesirable in the known tubes because thedischarge then passes over. anywhere between the anode and cathode in anuncontrolled manner, no ions are accelerated towards the .targetn andthe neutron yield drops substantially to zero. In general, thedisruptive discharge takes place in the form of a surface discharge onthe glass envelope In 11 itself, a low-pressure high-voltage disruptivedischarge has the desired property that a large number of positive ionsare formed without any separate ion source being necessary Thisadvantage is offset, however, by

a very serious disadvantage which hither-to made working with disruptivedischarges appear impossible. Here the ionization is not effected mainlyby electrons as in disruptive discharges in which the product ofpressure and electrode spacing lies above the Paschen minimum, but byions because the working range lies below the Paschen minimum. Themajority of ions are therefore produced in the vicinity of the cathodeand the ions which finally impinge on the target cathode have mostlypassed through only small differences in potential and therefore havesmall effective cross-sections for reaction. The yield of the nuclearreactions mentioned at the beginning is actually very greatly dependenton the accelerating voltage in the voltage range which is available inpractice and the maximum is only at an ion energy of about 2 m.e.v.

Another desirable characteristic of the disruptive discharge is itsshort duration. The known pulsated neutron sources all require more orless expensive electronic pulse sources.

It is the object of the invention to provide an arrangement for theproduction of neutron pulses which arrangement utilizes the advantageousproperties of low-pressure high-voltage disruptive discharges butlargely avoids their disadvantages. As a result, it is possible toproduce a particularly simple neutron source which enables intensiveneutron pulses of very short duration to be produced. It is true thatwith a constant ignition voltage, it is impossible to exert a directinfluence on the components of the energy spectrum originating fromhigh-speed ions and neutral particles. In itself, as high an ignitionvoltage as possible is desirable, but the height of the ignition voltageis limited by technical high-voltage considerations and economicconsiderations and possibly also by questions of space because thedifficulties increase very rapidly when the voltage is raised aboveabout kv.

It has been found, however, that the portion of the energy spectrumoriginating from the electrons can not only be increased but alsoshifted to higher energies. In this case, secondary electrons which areformed at the target are utilized to a considerable extent in contrastto the known arrangements in which these secondary electrons are alwayskept away from the anode by special screening electrodes in front of thetarget.

An arrangement for the production of neutrons by bombardrnent of adeuterium and/or tritium target with deuterium and/or tritium ionscomprising an envelope containing deuterium and/or tritium under reducedpressure, an anode to which a high-voltage pulse can be applied and atarget connected to a cathode, is characterised according to theinvention in that the envelope geometry is selected in such a mannerthat the discharge in the form of a low-pressure high-voltage disruptivedischarge starts at least substantially centrally on the target and thatthe anode is of hollow construction, is provided with an aperture at itsend facing the target and is shaped in such a manner that the secondaryelectrons formed at the target and the electrons formed in the dischargein front of the target are accelerated towards the anode by the highvoltage prevailing between the anode and cathode and are focused intothe interior of the anode, and the ions produced by these electrons inthe interior of the anode are withdrawn from the anode by the fieldreaching through the aperture into the interior of the anode and areaccelerated towards the target.

As a result of the fact that the anode is hollow in construction and isprovided with an aperture at the end on the target side, the electronsformed at or in front of the target can travel over considerably longerpaths and so form a large number of fresh ions substantially at anodepotential. By these means, the energy spectrum of the ions impinging onthe cathode is given a second lower maximum which lies substantially atthe maximum possible energy (which corresponds substantially to theignition voltage).

The current supply section for the tube according to the invention canbe designed in a very simple manner; it only needs to comprise acapacitor which can be charged to a high voltage and can be connected tothe tube through a switch, in the simplest case a spark gap. Since, incontrast to the known arrangements, the tube according to the inventionis operated with a disruptive discharge, there are considerably fewerrestriction with regard to the operating voltage and higher voltage canbe used more easily if only because of the simplicity of thearrangement. a

The invention will now be explained in more detail in connection withthe drawing which shows examples of embodiments which should not beinterpreted to limit the scope of the invention. In the drawing FIGURE 1shows an arrangement constructed according to the invention for theproduction of high-intensity short neutron pulses;

FIGURE 2 is a graph of the energy distribution of the ions bombardingthe target during a disruptive discharge, and FIGURE 3 shows a furtherexample of an embodiment of a discharge tube according to the invention.

The arrangement illustrated in FIGURE 1 comprises a glass envelope 1 inthe form of a hollow cylinder into one of which is introduced a cathode3, which is curved inwardly in the form of a cup and carries a target 2,and into the other end of which an anode 4 is introduced in a vacuumtight manner. The anode 4 is narrowed like a diaphragm at the end 5adjacent to the target, as will be described in more detail hereinafter.Inside the anode 4 there may be a reflector rod 6 which is electricallyconnected to the anode at the end of the tube remote from the target andwhich carries at its end of the tar-get side a head 7 which is concavetowards the front.

The anode 4 is surrounded by a glass sheath 8 which is formed like adiaphragm at its end wall between the anode and cathode.

The external shape of the front end 5 of the anode is determinedprimarily by considerations concerning highvoltage techniques, and theradius of curvature must not be too small lest premature disruptivedischarge should occur as a result of field emission. The size of theaperture at the front of the anode is selected in such a manner that, onthe occurrence of a disruptive discharge, a tubular plasma of sufficientdiameter is formed to cover a sufiiciently large area of the target 2which is secured to the cathode 3. In addition, the aperture shouldallow the cathode field to extend to a certain extent into the anode asshown by the equipotential areas indicated by broken lines.

The diameter of the front aperture in the sheath 8 is selected equal toor somewhat smaller than the diameter of the anode aperture. On the onehand, the glass sheath 8 serves the purpose of preventing the dischargefrom starting at an unwanted point on the outside of the anode and, onthe other hand, it acts as a lens electrode because it is negativelycharged by the secondary electrons emerging from the target.

In selecting the spacing of the individual parts of the tube, thefollowing conditions should be taken into consideration: The distancebetween the front end 5 of the anode 4 and the target 2 mounted on thecathode 3 should be as small as possible in order to keep as low aspossible the effects of charges in charge of the ions accelerated by theanode to the target. The lower limit is determined by the setting in offield emissions. The distance between the anode 4 and the glass sheath 8must be small enough to be able to prevent the discharge from startingexternally on the cylindrical part of the anode and, on the other hand,should not be too small because otherwise field emission may occur.

The distance between the sheath 8 and the outer wall 1 is determined bysimilar considerations: if the distance is too great the dischargestrikes upwards between the sheath and the envelope of the tube to theupper terminal electrode 11; if the distance is too small, unwanteddischarges or disruptive discharges may occur in the space in between,caused by wall charges and field emission. The wall thickness of thesheath must correspond at least to the dielectric strength for theoperating voltage.

Finally, these considerations also apply to the cathode 3. The distancebetween the cylindrical wall of the cathode and the glass envelope llmust be so narrow that the discharge cannot start at the cathode betweenit and the glass envelope. In order to prevent the discharge fromstarting in the region of the fusing of the cathode to the glassenvelope, the cylindrical part of the cathode must be suificiently long.The fact must also be taken into con sideration that target material isvaporized during the discharges and may accumulate at the diaphragm-likec0nstriction of the glass sheath so that field emission could occur atthe glass sheath. In the most unfavourable case, charges may form on theglass walls which are at the full anode or cathode potential.

A tube constructed in practice had the following dimensions althoughthese are not to be interpreted as being restrictive:

Length of the anode 4 About 300 mm. Diameter of the anode About 54 mm.Diameter of the anode aperture About 25 mm. Diameter of the sheathaperture About 25 mm. Wall thickness of the sheath About 5 mm. Length ofthe cathode (axial) About mm.

Spacing: Millimeters Anode 4-sheath 8 (radial) 3 Anode 4sheath 8 (axial)45 Sheath S-envelope 1 3-4 Target 2-sheath 8 (axial) 17 Cathode3-envelope 1 (radial) 10 The diameter of the glass diaphragmsubstantially determines the diameter of the tubular plasma being formedand hence the thermal loading of the target zone at which the dischargestarts. The diameter of the sheath aperture is preferably smaller thanthat of the target and that of the anode.

During the high-voltage disruptive discharge, electrons, most of whichare released from the cathode by the bombarding ions, fly into thehollow anode 4. Because of the length of path extended into the anodeand the probability of ion formation is increased and the electronsproduce ions substantially at anode potential. However, the ions must beprevented from being produced at such a great distance behind the anodeaperture that the field extending from the cathode into the anode is nolonger able to withdraw the ions out of the anode and to accelerate themtowards the target. In order to ensure this, the depth of the anodecavity is limited. The anode cavity is preferably spherical in shape orhas the shape of an ellipsoid of revolution. In the embodimentillustrated in FIGURE 1, this shape is approximately obtained 'by theconfiguration of the inner wall at the end of the anode at the targetside and by the reflector rod 6 which extends coaxially forwardly fromthe terminal electrode 11 into the anode and which terminates in aconcave end at the front. The electrons impinging on the anode and there flector rod are difiused back and are subjected to multiplereflection in the course of which their probability of ionization isparticularly great because of the relatively low energy and the greatlength of path.

The reflector rod 6 not only serves for the back diffusion of the partof the electrons which are focussed into the hollow anode but alsoco-acts with the hollow anode to form a high-frequency tank circuitwhich is excited to natural oscillation by the voltage variationsbetween the anode, and cathode and by is:higher, than the disruptivevoltage of the tube.

odeat the target; 1 not involve the spatial; restrictions which arecaused by the inwardly curved oathode 3 of FIGURE 1.

interaction with the injected electrons: The high-frequency alternatingfield developing in the vicinity of the anode aperture lengthens theelectron paths and accelerates slow electrons.

In addition, an annular electrode 13, which is electrically connected.to the cathode 3, may be provided externallyi on the glass envelope 1.This electrode produces anelectrical field extending radially in theregion of the front: end of the anode and so contributes to theprevention of the development of surface discharges between the fusionpoints :of the anode and the cathode. The annular electrodefl 13 may beconnected to the cathode 3 through aninductancez: 12 so that anoscillatory circuit is formed which; circuit is preferably. tuned to thefrequency of the tank circuit formed-by=the anode 4 and reflector rod 6.

The tannular. electrode 13. and the mentioned high-frequency oscillatingcircuits also lead to a reduction in the ignition pressure which ishighly desirable because of the associated .reduction in changes incharge phenomena.

By. this means the ignition pressure could be reduced for example, from0.14 torr to 7X torr of the deuterium filling with an operatingvoltageof 150 kv.

i which: can be charged to a high voltage, for example 150 kv. or moreby means of a known arrangement which is not illustrated, is connectedto the anode and cathode through: a high-voltage switch, in the simplestcase a spark gap 10. The ignition voltage of the spark gap 10 In thesimplest case, the capacitor 9 may consist of a plurality of. lengths ofhigh-voltage cable connected in parallel. The inductance of the supplyconductors should be kept as low as possible if short neutron pulses arerequired.

FIGUREZ shows the energy spectrum during a disruptive, discharge. Thenumber N of the ions is plotted in arbitrary units as ordinates againstthe energy E with which; the ions impingeon'the target. The maximumpossible; energy (ignoring any superimposed highfrequency oscillations)corresponds to the ignition voltage Z. 1. The curve shown in brokenlines indicates the energy distribution for a disruptive discharge in aknown tube while the full lcurve shows the energy distribution of theions, in a tube according to theinvention. It will be seen that in thevicinity of themaximum energy Z, the distribution curve forms a maximumwhichis responsible for the neutrons produced in the tube. FIGURElBshows another example of an embodiment of a tube in which the target2'is concave in construction and is mounted on the inner wall of themetallic cathode 3 which forms :a part of the tube envelope. The cathodecap is fused to a glass cylinder 1 further up in known manner.Otherwise. the parts of the tube, only part of which is illustrated,correspond to those in FIGURE 1; corresponding reference numerals areprovided with, a prime. i

i In operation, the disruptive discharge takes place over thelongestlpossible path, that is to say between the front end 5' of; theanode andthe concave target 2', corresponding to the left-hand region.of the Paschen curve. The secondary electrons formed are focussed by theconcave targetlinto the anode aperture. A flashover between the cathodeand the :upperterminal electrode of the anode,

which is not illustrated,,is prevented by the short distance between thanode. and cathode and between the anode and the glass enveloperespectively. The distances should not, however, be so short that thereis a risk of field emission.

The substances to be irradiated with neutrons can be arranged round: thetube or immediately; outside the cath- The tube illustrated in FIGURE 3does Naturally, a concave reflector rod may also be used in FIGURE 2instead of the plane reflector rod illustrated. The interior of theanode may be lined with titanium or another material which absorbsdeuterium and then acts as a deuterium store. Deuterium ions are thenreleased pulse-wise as a result of the heating caused by the disreuptivedischarge. The arrangement in FIGURE 1 may likewise comprise a targetwhich is concave in respect of the anode in order to focus the secondaryelectrons into the anode aperture.

In order to increase the probability of ionization in the anode cavitystill further, magnetic fields may be provided in a manner known per se;thus it is possible, for example, to construct the stem of the reflectorrod 6 in the form of a permanent magnet or to construct the head of thereflector rod from a permanent-magnet material with a sufliciently highCurie point.

The tubes described render it possible to produce very short neutronpulses of high intensity with a minimum expenditure; for example, theduration may be between about 10- seconds and 10- seconds and theneutron peak source about 10 neutron sec." in D(dn)He reactions andabout 3X10 neutron sec.- in T(dn)He reactions. Since no complicatedelectronic devices are necessary for operation, the operating voltagecan be raised to the extent permitted by economic considerations, thehigh-voltage sources available and insulation problems. What we claimis:

1. A device for the production of neutron pulses by bombardment of atarget with ions, comprising, in combination:

(a) an envelope;

(b) a free gas in said envelope under reduced pressure and adapted for agas discharge;

(c) a target in said envelope;

(d) a hollow anode in said envelope spaced from said target and havingan aperture facing said target;

(e) a cathode spaced from said anode and on which said target ismounted;

(f) means for applying a high voltage pulse between said anode and saidcathode for producing a lowpressure high-voltage disruptive dischargebetween said anode and said cathode; and

(g) means for focussing into the interior of the anode the electronswhich are formed at the target and in the region of the discharge infront of the target.

2. A device as defined in claim 1, wherein said means for applying ahigh voltage pulse between said anode and said cathode includes acapacitor which is periodically charged to a high voltage and thendischarged, and a high voltage switch means for connecting saidcapacitor, which is charged to a voltage which is higher than thebreakdown voltage between said anode and said cathode, directly acrosssaid anode and said cathode.

'3. A device for the production of neutron pulses by bombardment of atarget with ions, comprising, in combination:

(a) an envelope;

(b) a gas in said envelope under reduced pressure and forming an ionsource;

(c) a target in said envelope;

(d) a hollow anode in said envelope spaced from said target and havingan aperture facing said target and to which a high voltage pulse can beapplied; (e) a cathode spaced from said anode and on which said targetis mounted;

(f) means for producing a low-pressure high-voltage disruptive dischargein said envelope; and

(g) means for focussing into the interior of the anode the electronswhich are formed at the target and. in the region of the discharge infront of the target, said focussing means including the target beingconcave with respect to the anode so that secondary electrons formed onthe target are focussed into the aperture in said anode.

4. A device for the production of neutron pulses by bombardment of atarget with ions, comprising, in combination:

(a) an envelope;

(b) a gas in said envelope under reduced pressure and forming an ionsource;

(c) a target in said envelope;

(d) a hollow anode in said envelope spaced from said target and havingan aperture facing said target and to which a high voltage pulse can beapplied;

(e) a cathode spaced from said anode and on which said target ismounted;

(f) means for producing a low-pressure high-voltage disruptive dischargein said envelope; and

(g) means for focussing into the interior of the anode the electronswhich are formed at the target and in the region of the discharge infront of the target, said focussing means including a glass sheathsurrounding said anode, spaced from said envelope and having an apertureadjacent to and coaxial with said anode aperture.

5. A device as defined in claim 4 wherein said glass sheath surroundsthe end of the anode having the aperture as a diaphragm and the glasssheath aperture is at most equal to the anode aperture.

6. An arrangement as claimed in claim 1, characterised in that theinterior of the anode is at least approximately an ellipsoid ofrevolution in shape and that its depth is such that the ions produced inthe interior are accelerated towards the cathode by the voltage set upbetween the anode and cathode.

7. An arrangement as claimed in claim 1, characterised in that the anodeis a hollow cylinder and is narrowed like a diaphragm at the front end(5) adjacent to the target.

8. A device for the production of neutron pulses by bombardment of atarget with ions, comprising, in combination:

(a) an envelope;

(b) a gas in said envelope under reduced pressure and forming an ionsource;

() a target in said envelope;

(d) a hollow anode in said envelope spaced from said target and havingan aperture facing said target and to which a high voltage pulse can beapplied;

(e) a cathode spaced from said anode and on which said target ismounted;

(f) means for producing a low-pressure high-voltage disruptive dischargein said envelope;

(g) means for focussing into the interior of the anode the electronswhich are formed at the target and in the region of the discharge infront of the target, said anode being a hollow cylinder and narrowedlike a diaphragm at the front end adjacent to the target; and

(h) a reflector rod arranged substantially coaxially in the interior ofthe anode cylinder.

9. A device as defined in claim 8 wherein said envelope is glass andfurther comprising an annular electrode mounted externally on the glassenvelope in the region of the anode aperture and electrically connectedto said cathode.

10. An arrangement as claimed in claim 8, characterised in that thereflector rod terminates in a concave disc.

11. An arrangement as claimed in claim 7 characterised in that the innerwall merging into the aperture in the front end of the anode is shapedin such a manner that it approximates a part of a spherical surface.

12. An arrangement as claimed in claim 1, characterised in that thecathode is in the form of a cup (3) which extends into the interior ofthe envelope (1) and the end of which carries the target (2).

13. An arrangement as claimed in claim 9, characterised in that aninductance (11) is inserted in the connection between the annularelectrode (13) and the cath- 8 ode (3) and that an oscillatory circuitwhich is thus formed is preferably tuned to the frequency of a tankcircuit formed by the anode cylinder (4) and the reflector rod (6).

14. An arrangement as claimed in claim 1, characterised in that thecathode (1) forms a portion of the envelope and surrounds the end (5) ofthe anode (4) at the target side; that the end of the cathode on theanode side is fused to one end of an insulating tube (1') to the otherend of which the anode (4') is fused; that the target (2') mounted onthe inner wall of the cathode is concave in construction and that theradial distance between the anode and the cathode and the configurationof the end (5) of the anode on the target side are so small that thedisruptive discharge takes place between the front end of the anode andthe target.

15. A device for the production of neutron pulses by bombardment of atarget with ions, comprising, in combination:

(a) an envelope;

(b) a gas in said envelope under reduced pressure and forming an ionsource and being one selected from the group consisting of deuterium andtritium;

(c) a target in said envelope and being one selected from the groupconsisting of deuterium and tritium;

(d) a hollow cylindrical anode in said envelope spaced from said targetand having an aperture facing said target and to which a high voltagepulse can be applied;

(e) a cathode spaced from and opposite to the aperture in said anode andon which said target is mounted;

(f) means including a capacitor for periodically producing alow-pressure high-voltage disruptive discharge in said envelope; and

(g) means for focussing into the interior of the anode the electronswhich are formed at the target and in the region of the discharge infront of the target, and including a glass sheath surrounding said anodeand having an opening adjacent to and coaxial with said anode opening.

16. A device for the production of neutron pulses by bombardment of atarget with ions, comprising, in combination:

(a) an envelope;

(b) a gas in said envelope under reduced pressure and forming an ionsource and being one selected from the group consisting of deuterium andtritium;

(c) a target in said envelope and being one selected from the groupconsisting of deuterium and tritium;

(d) a hollow anode in said envelope spaced from said target and havingan aperture facing said target and to which a high voltage pulse can beapplied;

(e) a cathode spaced from said anode and on which said target ismounted;

(f) means including a capacitor for producing a lowpressure high-voltagedisruptive discharge in said envelope; 'and (g) said target beingconcave with respect to said anode for focussing into the interior ofthe anode the electrons which are formed at the target and in the regionof the discharge in front of the target.

17. A device as defined in claim 1 wherein the free gas is deuterium ortritium.

References Cited by the Examiner UNITED STATES PATENTS 9/1959 Graves etal. 313-61 7/1964 Carr 250-845

1. A DEVICE FOR THE PRODUCTION OF NEUTRON PULSES BY BOMBARDMENT OF ATARGET WITH IONS, COMPRISING, IN COMBINATION: (A) AN ENVELOPE; (B) AFREE GAS IN SAID ENVELOPE UNDER REDUCED PRESSURE AND ADAPTED FOR A GASDISCHARGE; (C) A TARGET IN SAID ENVELOPE; (D) A HOLLOW ANODE IN SAIDENVELOPE SPACED FROM SAID TARGET AND HAVING AN APERTURE FACING SAIDTARGET; (E) A CATHODE SPACED FROM SAID ANODE AND ON WHICH SAID TARGET ISMOUNTED; (F) MEANS FOR APPLYING A HIGH VOLTAGE PULSE BETWEEN SAID ANODEAND SAID CATHODE FOR PRODUCING A LOWPRESSURE HIGH-VOLTAGE DISRUPTIVEDISCHARGE BETWEEN SAID ANODE AND SAID CATHODE; AND (G) MEANS FORFOCUSSING INTO THE INTERIOR OF THE ANODE THE ELECTRONS WHICH ARE FORMEDAT THE TARGET AND IN THE REGION OF THE DISCHARGE IN FRONT OF THE TARGET.